The Early 'Beginnings' of Light?

[ATF Ravok]

For Asha'man and others like him. =)

I am just curious about cosmology (again), and in my imagination I wonder what is the minimal chemical composition || process needed in order to create a spark of light. My curiosity stems from the Big Bang Theory. I mean, the idea is that everything (at the time) decided to have a Pow Wow and thus, Kaboom! We have a new universe!

I am trying to understand where in that process does the 'stuff' start mixing enough to generate that first spark of brightness.

So, what elements under what conditions do you think (or know) would be the first ones to shine?

[XMEN Gambit]

Well, light = photons. You probably knew that.

The most common way we know of to generate photons is to "excite" an atom with enough energy (heat, radiation, or electricity) that its electron shells "expand" into a higher "orbit," and then when they accumulate enough energy they collapse back to where they belong and emit a photon. Forgive my oversimplification, Asha. Sometimes radioactive elements also emit photons along with other particles, but I don't remember if that's a primary or secondary effect.

I think just about any element under the right conditions could be made to radiate light, but I'm not sure.

[XMEN Ashaman DTM]

Technically, it's the electrons that spontaneously emit the photons to shed their excess energy. The nucleus can emit photons, but the energy states of an atom are defined by where the electrons are found (higher energy atoms have more energetic electrons). Also, the electrons are moving around the nucleus so fast that they effectively act as a screen against most photons getting to the nucleus.


The simplest way to think of a photon is that it's a little bundle of energy. And the creation process is simply the way that atoms (electrons to be more precise) give off energy. Chemical processes are processes that involve how the electrons of an atom interact with each other, and in some chemical reactions, you can get photons.

But it all boils down to how the electrons are behaving in the atom. The amount of energy in a photon also tells you the wavelength of that photon. The more energetic ones are shorter in wavelength: radio, infrared, visible light, UV, Xrays, gamma radiation. Those are the names we have given to different sections of the electromagnetic spectrum; it's all photons, but at different energies.


In chemical reactions, the electrons are slowing down, moving from one atom to another, losing/lowering their energy when they get into a more stable "orbit" about the nucleus. In lasers (and other radiation-type processes), you are putting energy into electrons (both by themselves in some lasers, and as they "orbit" nuclei in most lasers), and the electrons are giving up that energy to maintain their stability with their surroundings. In nuclear reactions (fission and fusion) you can get packets of energy (photons) thrown off as a result of a reaction. And in radioactive decay, the energy can be shed from the atoms as high energy photons (gamma radiation), electrons boiling away (beta radiation), or by chunks of nuclei being a produt of a reaction (alpha radiation). Of the three, only gamma is the most worrysome (think random cosmic rays passing through you and changing your DNA as they do because they hit atoms and knock things off). Alpha and beta are both stopped by your skin, but both can give burns, and beta is the stronger of the two and it penetrates your skin a few millimeters (a 1/8 of an inch or so).

When two things mix, they will always try to come to what is called equilibrium, and that equilibrium will be a point of stability for that mixture. It's one of the laws of the universe that things will always try and settle down. In the process of a reaction settling down, photons can get created. And all that the photon is, is the embodiment of the energy left over after the reaction has taken place. By that I mean that there are very few reactions that take place that result in a break-even; there is almost always a tiny bit extra energy left over that physicists and engineers always treat as excess heat, but it's actually a photon that goes off on its own in some direction.


And just like everything else in the universe, there are certain probabilities of things happening. It's more likely that the matter that makes up you and your chair will never suddenly pass through each other. But it's an extremely remote possibility, and this phenomenon is seen in what is called "quantum tunneling" where energy and matter don't overcome some barrier keeping them where they are. They "tunnel" through the barrier. These "more likely events and processes" are what we see every day. The sun could suddenly stop doing fusion, but it's not likely to happen with what we know about the sun. A reaction could produce many photons of equal energy instead of just one big one, but it's not likely to happen, because that's not the minimum energy state.


All you really need for that one single photon is to have one single electron become excited and have it put or exist in an environment that is at a much lower energy level than it is. In deep space, the average temperature is 3 Kelvin (that's 3 degrees Celsius above the coldest possible temperature in the universe). So an electron sitting in deep space will naturally want to be at 3 Kelvin, so it will spontaneously emit the right amount of energy to get into that average state. Temperature is technically an average of all the particle speeds in a certain volume. And particle speeds are measures of the particle energies. So yes, you can talk about electron temperature and make perfect sense.



That's probably a bit of a novel, but it's basically the same idea: things (systems/ particles/ electrons/ suns/ people/ etc) all want to move towards a minimum energy state. And for electrons, the way for them to get there is to give up energy. And almost always that energy comes in the form of a photon.

[ATF Ravok]

I love asking you guys questions: I always learn twice more than I thought I could. =)

Thanks a lot for the replies!

[XMEN Iceman]

Asha,
One of my best friends is a Dr. Chuck Hass. He has several honorary degrees in Lasers. Awesome guy. But his specialty is Metrology, or calibration of measuring instruments. Anyway, he has gotten me interested in some of the sciences he works with.

My question to you is...lasers...I don't quite understand how you get the photons excited enough...then get them to bounce back and forth until you can "focus" them into the beam. Do you excite them in a mirrored chamber until you choose to open an end? etc.

My curiosity is showing.

[XMEN Ashaman DTM]

Well...

There are many ways of actually doing it, but the process is pretty much straightforward: get the electrons in the material you are working with to undergo a "population inversion" (making more electrons excited than at ground or rest state), the electrons will spontaneously emit radiation when they choose, you keep the photons radiated from the electrons in a resonator cavity with mirrored (reflective) surfaces, and one of those mirrors wil be partially transparent to allow some photons through (depends on the laser, but 10% is not unreasonable). Then the photons are focused using mirrors and lenses. The focusing process will cause the beam cross-section (the beam waist) to converge and then diverge.


You want a partially transparent mirror to allow some of the photons to escape, but not all of them, because you need a lot of photons in the resonator cavity to bounce around and hit other electrons to keep the process of maintaining the population inversion going. The photons will have an energy associated with their creation (the spontaneous emission stuff), and an apparent energy due to relative velocity as the photon interacts with an electron. In some collisions, the photon will hit the electron with a velocity that is coming towards the electron which means the electron will "see" a photon coming at it that is blue shifted (shorter wavelength due to Doppler shift... just as an oncoming train has a high-pitched horn), that blue shift means that the electron "sees" a higher energy and is likely to generate two or more photons when it is struck. There is no energy gained or lost, that kinetic energy of the incoming photon is represented by the apparent shortening of the wavelength as seen by the electron. In other collisions, the electron will be hit by another electron and give off energy in the form of a photon.

All of that collision stuff is happening inside the resonator cavity. The resonator cavity is important because that's what makes the photons coherent. Think of a string tied between two trees. You can make the string oscillate and it will only have certain frequencies of oscillation because the ends are fixed. In a resonator for electromagnetic energy, the length determines what wavelengths of energy you can have in it. It will always be c (speed of light) divided by L (the distance between the walls of the cavity) that gives you the frequency of the energy inside. The wavelength, L, will always be the most common, and then shorter wavelengths according to how much energy you are putting into the cavity. Without the resonator cavity, you would just have a brightly lit material giving off photons. Say a cloud made of Neon gas, without the resonator cavity, would just be a glowing reddish colored cloud. With the resonator cavity, you have coherence and thus, a laser beam.


Now as for focusing, you can only focus down to the wavelength of the beam itself. You can't get any smaller because the physical limit to the packet of energy is tied to the wavelength of the photon. To get a smaller focus cross-section, you would have to use a higher energy photon (thus a shorter wavelength). And you can never really reach the smallest cross-section because before you do, your beam will run into whatever molecules are floating in its path. Even if you have a vacuum chamber that is reading a perfect vacuum, you will still have atoms and molecules floating around in it. What that does is those photons in your beam will focus down, as you want, and the photons will interact with the electrons in the atoms or molecules that are in the way. This does two things: first the electrons get excited and second, the electrons can get knocked off when they are excited. In a laser the electrons can be "boiled off" the atoms and what you get is a plasma. What happens next is that you will see arcing as the electrons are pushed apart by the laser, and then recombine with the lonely atoms/molecules to equalize the charge.

If you ever see this, it's a pretty neat thing to watch. I've seen it in a video of a lab experiment that was showing how energy is quantized and the limit to how focused a laser can get is about one wavelength of the laser's photons. In some things, this plasma creation is a good thing. But not when you are trying to focus a laser.

[XMEN Ashaman DTM]

Also, there are many ways to get photons. Chemical reactions and irradiating materials are the most common. If you could set up a resonator cavity that contained Helium and Neon, for example, and you put an electric field into the gas cloud (via a flashlamp, a light source, or a RF coil), you could stimulate the He-Ne gas and if you kept stimulating it, you would continue to generate photons. It's just a matter of getting the electrons to "snap" in terms of their energy. You want a quick increase, with enough energy that the chance of getting photons is a good one.


The hardest way to generate photons though is using electrons directly. You have to do it in a particle accelerator, and you have to use magnetic fields to bend the electrons (which is essentially accelerating them around a radius), and that acceleration causes them to gain energy (acceleration x distance is the same thing as energy; think work = energy = force times distance). When the electrons get excited, they want to get back to their rest state, which means the electrons give off photons.

(Do a google search for Free Electron Lasers to get an idea of what is going on in the above paragraph.)

You just have to remember that it's the electrons that give off photons. Sometimes those electrons are trapped in atoms, sometimes they're free.